/Engine/Source/Runtime/AnimationCore/Public/BoneWeights.h
Return the weight at indexIndex. Using an index value less than zero  or equal to or greater than the result ofNum()is an undefined operation.
The bone weight value for that index, or undefined if outside the valid range.
const [FBoneWeight](API\Runtime\AnimationCore\FBoneWeight) & operator[] ( int32 Index ) const